This invention relates to communication data buses. In particular, the invention relates to optical multiplex data buses, and to the monitoring of electro-optical transducers used to communicate with the bus.
Present multiplex data bus technology using electrical conductors is exemplified by the bipolar Manchester-encoded bus, which is well-known in the art. While this system performs adequately in systems with data rates of up to 10 MHZ, efforts must be expended to guarantee its performance in certain environments such as in aircraft. Some of the problems encountered are ground loops, radiated and induced noise, amplitude variation and the cost and weight involved in coupling different devices to the wire cable. In order to solve some of these problems, optical multiplex buses have been developed. The optical bus substantially reduces noise, cross talk, and ground loops. Other advantages include freedom from intermittent connections and short circuits, an ability to electrically isolate interconnected redundant systems and safety in explosive environments. While early optical multiplex bus applications have attempted to imitate standard bus design, such as the MIL-STD-1553 bus, later applications sought to utilize the greater band width, lighter weight, and potentially lower cost of electro-optic technology.
Electro-optic communication systems contain three basic blocks:
(1) a transmitter which converts electrical signals to light. The signals may be at different logic levels for digital communications. PA1 (2) An optical cable which couples the light signals to various transmitting and receiving systems. PA1 (3) A receiver which converts light from the cable to electrical signals, including logic levels for digital communications.
It is desirable that the system be monitored, particularly at the transmitter, in order to determine whether signals are being properly transmitted by the transmitter. This would allow fault isolation between systems and alert an internal computer or other monitor so as to signal failure in transmission. Typically, the transmitter is a light emitting diode (LED) or laser diode, driven by a driver amplifier. The principle failure modes for both the diode and the driver are open and short circuits. The present invention is directed to circuitry which is able to monitor circuit conditions which appear between the driver and diode when the transmitter is operating.
It is accordingly an object of the present invention to provide a more reliable and efficient form of data communication between electronic systems and components. This is particularly significant in data buses used on aircraft having numerous electronic systems on board. It is further desired to provide a communication system such as an optical data bus which provides a higher data rate than a conventional Manchester-encoded wire pair electrical multiplex bus, in which the reliability of transmission through the data bus is assured. It is a further object to provide a fault isolation system for use with transmitters for an optical data bus.